<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>行内元素</title>
</head>
<style>

    #app{
        width: 200px;
        height: 200px;
        background-color: #bbffaa;

    }
    span{
        background-color: red;
    }
    #a{
       margin-top: 20px;
        width: 50px;
        height: 50px;
        opacity: .5;
        background-color: rgba(0,0,0,.5);

    }
</style>
<body>
    <div id="app">
        <span id="a">123</span>
        <span>456</span>
        我是垂直文字
    </div>
    <!--    行内元素垂直方向下不生效,水平方向下可生效,而且也没有固定的宽高-->
</body>
</html>
